Metallic beverage can end closures have historically been designed and manufactured to provide a stiffening bead referred to as countersink. This feature may include vertical walls attached by a full radius bottom forming a channel, and in some embodiments may incorporate arcuate shapes or other geometric profiles. Absolute vertical walls may not exist, but generally the more vertical they become the greater the resistance to deformations resulting from internal pressure.
Beverage can bodies and end closures must be durable to withstand high internal pressures, yet manufactured with extremely thin and durable materials such as aluminum to decrease the overall cost of the manufacturing process and the weight of the finished product. Accordingly, there exists a significant need for a durable beverage can end closure which can withstand the high internal pressures created by carbonated beverages, and the external forces applied during shipping, yet which are made from durable, lightweight and extremely thin metallic materials with geometric configurations which reduce material requirements. To obtain these characteristics, can end closures require aggressive material working to achieve the various forms and geometries, which is generally accomplished utilizing a male/female tool combination. Unfortunately, this process may lead to inconsistencies within a given contour or geometry. Formation inconsistencies also apply to strength performance. The aggressive forming within the countersink may alter other characteristics within the body of the entire structure. Thus, there is a significant need to provide an apparatus and material forming technique which provides improved end closure on container geometries which have improved strength and buckle resistance. These features are obtained in one embodiment by placing the end closure material in compression during forming to avoid thinning and unwanted material deformation, while simultaneously supporting certain portions of the end closure chuck wall and seaming crown geometry during forming while not supporting other portions to create a predetermined shape.
One patent related to a method and apparatus for producing a container end closure countersink is described in U.S. Pat. No. 5,685,189, (the “'189 patent”) which is incorporated herein by reference in its entirety. In the '189 patent, a portion of the countersink is formed when the countersink is unsupported by tooling while the countersink is placed in compression. Unfortunately, with lighter gage stock materials this process has been found to allow unwanted deformation in the chuck wall and seaming crown, and thus inconsistencies in the end closure geometry.